This invention relates to a method and apparatus for tracking control and, more particularly, to a method and apparatus for controlling the movement of a record medium having skewed tracks thereon which are scanned by a transducer when the medium is commanded to move at a speed and distance which are variable, such that the transducer accurately scans, or tracks, the record tracks.
Video recorders are used to record video signals on a record medium, such as a magnetic tape, a magnetic sheet, a magnetic disc, an optical video disc, a capacitive video disc, and the like, which video signals subsequently are reproduced and used, for example, to broadcast previously recorded video programs. Typically, special effects are obtained by reproducing the video signals at different playback speeds, such as fast-motion, slow-motion, still-motion and reverse-motion speeds. Also, in producing a video program, it often is necessary to edit various segments or portions so as to formulate a complete program. Typically, such editing is facilitated by advancing (or reversing) the record medium at slow speeds when searching for the optimum location at which edit points should be made. Once those points are ascertained and detected, information from one record medium may be transferred to another and either inserted into a "slot" or merely assembled so as to form the desired video program.
Techniques which have been developed for video signal recording, reproducing and editing, as well as the apparatus used therewith, have been adapted for other disciplines. For example, it has been proposed to use a video recorder to record digitized audio signals on the record medium. It also has been proposed to use a video recorder to record other digital information signals. The relatively high frequency band and high density of video recorders results in advantageous uses thereof for the foregoing as well as other purposes.
In a typical video recorder, such as a video tape recorder (VTR), a magnetic tape is helically wrapped around at least a portion of the periphery of a guide drum and is transported at a fixed speed for video recording. The guide drum typically includes a rotary section to which one or more recording transducers, or heads, are mounted. The longitudinal path along which the tape is advanced is at an angle with respect to the rotary path followed by the heads, thus resulting in successive, parallel, skewed tracks across the tape. During a recording operation, the tape speed as well as the rotary speed of the heads are servo-controlled such that uniform tracks of substantially constant pitch are recorded. It has become conventional to record a single field of video signals in each track and, in one format, the usual vertical synchronizing signals (referred to herein as the vertical sync signals) are recorded at the end of each track. Also, in a preferred recording format, the successive horizontal synchronizing signals (referred to as the horizontal sync signals) are aligned with one another from one track to the next. This alignment is the so-called H-alignment and minimizes undesired cross-talk from one track to the next due to the reproduction of non-aligned horizontal sync signals.
During the playback mode, if the tape is advanced at the same speed as during the recording mode, the servo system of the VTR is effective to bring the scanning trace of the reproducing transducer, or head, into coincidence with the previously recorded skewed tracks. Thus, for "normal" reproduction, each of the previously recorded record tracks is scanned accurately by the playback head or heads. However, during so-called non-normal playback modes, the speed at which the tape is transported differs from the recording speed, and this results in the scanning trace of a playback head to be at a discrete angle with respect to the track being scanned thereby. This angle is a function not only of the tape speed but also of the direction in which the tape is driven. Consequently, the video signals which are recorded in the successive tracks are not reproduced accurately.
To obtain accurate scanning, or tracking, of the previously recorded record tracks by the rotary playback head during non-normal, or "special effects" playback modes, various tracking control systems and techniques have been proposed. For example, in the tracking control systems described in U.S. Pat. Nos. 4,163,994, 4,172,264, 4,237,399, 4,287,538 and 4,296,443, the playback head is mounted on a deflecting device, generally referred to as a bi-morph leaf, which deflects in response to a drive voltage supplied thereto so as to correspondingly deflect, or displace, the head. Deflections of the bi-morph leaf, which may be, for example, a piezo-ceramic material, thus may be controlled so as to urge the head into proper tracking alignment with the track being scanned, even though the normal trace of the head would not coincide with the track during special effect playback modes. Deflection of the bi-morph leaf is controlled by sensing the error between the actual scanning trace of the head and the track, and then adjusting the drive voltage supplied to the bi-morph leaf in a manner which reduces this error to a null value.
During the still motion reproduction mode, the tape is maintained stationary and the playback head scans the same track repeatedly. In this mode, the head is deflected during each scan, typically by a variable amount during the length of the scannning trace, to bring it into alignment with the stationary track. At the completion of one scanning trace, the head is brought back to its initial position so as to be in alignment with the beginning of the scanned track during the next scanning trace. During this mode of operation, the head is caused to "jump" or "fly-back"by the same amount at the end of each trace. Similarly, during slow-motion, fast-motion or reverse-motion modes, the head must be jumped, or caused to fly-back, at the end of each trace so as to be in proper position to scan the correct record track during its next trace. Of course, since the tape is transported during these modes, a head jump, or fly-back, at the end of every trace sometimes is omitted. For example, during slow-motion reproduction, after a number of successive traces of the same track, the tape movement will be such that the next track is brought into position to be scanned. Consequently, rather than jump back to the beginning of the previously scanned track, the head is brought into position so as to scan this next track. This may be achieved by, for example, inhibiting the head jump or fly-back at that time. Also, during the fast-motion mode of reproduction, the tape may be moving at a speed such that, at the completion of the scanning of one track, the tape is sufficiently advanced that the head skips the next adjacent track and scans the following track. In scanning every other, or alternate, tracks, the aforementioned head jump, or fly-back, is not carried out.
If the tape is transported at a uniform speed, and the particular mode of signal reproduction is known, the angle, or slope, of the scanning trace may be corrected, i.e. brought into alignment with the track being scanned, by a drive voltage of, for example, constant slope. Then, at the completion of the scanning trace, this drive voltage is reset and a so-called head-jump voltage is applied to the bi-morph leaf to deflect the head into proper position for the next trace. It is desirable to apply a slope-correcting voltage, as well as a head-jump voltage, within predetermined limits so as to avoid over-driving, or overloading, the bi-morph leaf. Ideally, both the slope-correcting and head-jump voltages should be minimized, or at least maintained below a predetermined maximum limit. For example, during slow-motion reproduction modes, after a particular track is scanned a number of times, the tape may be sufficiently transported that the drive voltage next supplied to the bi-morph leaf might be too large if the same track is scanned again rather than if the next track is scanned. In this instance, it is desirable to inhibit the head-jump voltage and enable the head to scan the next-following track. Thus, proper control over the head-jump, or fly-back, is needed so as to avoid overloading the bi-morph leaf. In one head-jump control proposal, the drive voltage applied to the bi-morph leaf is detected; and when this drive voltage exceeds a predetermined amount, such as the amount which approaches the physical limit of the bi-morph leaf, the deflection of the head is controlled whereby the next-following track is scanned. Hence, the head is not caused to fly-back, which, if allowed, would result in a drive voltage that could overload the bi-morph leaf during the next scan. Unfortunately, the physical deformation limits of a bi-morph leaf generally change with age. Hence, the detected drive voltage might not be an accurate indication of the loading on the bi-morph leaf.
Another head-jump control technique is to determine when the head should be caused to fly-back as a function of the frequency and phase of the reproduced horizontal sync signals. The frequency and phase of these signals change as a function of the speed and distance at which the tape is transported. The deflection of the playback head during special effects modes results in a deflection component in the longitudinal direction (i.e., the direction of transport) of the tape. This component is equivalent to tape movement and, therefore, imparts changes in the frequency and phase of the horizontal sync signals which are reproduced by the head. Since these changes are due to the actual deflection of the head and not due to changes in the characteristics of the bi-morph leaf or its control circuitry, such changes in the frequency and phase of the reproduced horizontal sync signals provide an accurate indication of the degree of deflection and, thus, can be used to determine when a head-jump, or fly-back, should be carried out or inhibited.
Yet another head-jump control proposal is set out in copending application Ser. No. 06/347,486, now U.S. Pat. No. 4,445,146.
In the foregoing proposals for controllig head-jump so as to avoid overloading the bi-morph leaf, it is assumed that the tape is transported at a uniform speed during the special effect reproduction mode. While such uniform speed generally is attained in most instances, it is not unusual for the tape to be transported at a non-uniform, or irregular speed during an edit operation. A manual control may be provided whereby the tape is advanced or reversed irregularly, as in a stepping motion, by which the editor steps from field-to-field in order to ascertain a desired edit point. For example, a so-called "jog wheel" may be rotated by the editor so as to transport the tape in a direction determined by the direction of rotation of the jog wheel and at a speed determined by the speed of rotation of that wheel. Of course, even during such irregular movements of the tape, it is desirable for the head to track the previously recorded record tracks properly. Thus, the deflection of the head in response to drive voltages applied to the bi-morph leaf must be controlled so as to bring the scanning traces into coincidence with the scanned tracks. Unfortunately, the tape transport system exhibits electrical and mechanical time delays and inertia such that the actual movement of the tape is delayed from the commanded movement thereof, that is, the tape is delayed with respect to the operation of the aforementioned jog wheel. Such delays result in errors of the scanning traces with respect to the tracks. When the aforenoted tracking control systems are used during such edit modes, undesired noise and jitter are present in the video picture which is reproduced from the scanned tracks due to misalignment of the scanning traces with respect to such tracks.